a) Field of the Invention
The present invention relates to an air pressure variation detector which is adapted to be capable of detecting variations of air pressure by using a piezoelectric element and suited, for example, for use with alarm systems.
b) Description of the Prior Art
There are conventionally known various types of detectors to be used as the means for detecting whether or not the enclosed states of enclosed spaces are broken into, houses, automobiles, libraries, moneysafe rooms, refrigerating warehouses and so on. On the basis of on operating principles, these detectors are classified into electromagnetic type, light wave type, electric wave type and sound wave type.
Out of these detectors, the electromagnetic type detector is exemplified by a magnetic approach switch which is composed of a pair of magnets and a reed relay. This switch is used for detecting open/closed states of doors and windows of rooms of houses since it has a relatively simple construction and is inexpensive. However, this electromagnetic detector is incapable of detecting breakage of the wall, door or window on which it is mounted when the central portion thereof is broken, and is therefore unsuited, for example, to prevent robbery from automobiles for which windows of the automobiles are broken very often. Further, the light wave type, electric wave type and sound wave type detectors, which are examplified by the infrared sensor, microwave sensor and ultrasonic wave sensor, are capable of detecting breakage despite location thereof so far as it is caused by persons, aminals and machines, but have narrow field angles or detecting ranges, whereby these detectors must be installed in a plural number at one site and are obliged to be used only at restricted kinds of locations.
In the recent years, detectors having high sensitivities and convenient for use are increasingly demanded not only for detecting breakage of enclosed states of spaces delimited from external world but also for detecting open/closed states of doors for computerized management of egress and ingress from and into rooms for the neccessity of prevention of robbery of the so-called trade secret management. Since the above-mentioned electromagnetic type and sound wave type detectors, etc. cannot perform detection with sensitivity high enough for meeting these demends, researches are made to improve a pressure sensor which is composed of a combination of a pressure-sensitive element and a diaphragm, and capable of detecting vibrations of air.
From the viewpoint of practical use, however, there still remain technological problems in said pressure sensor for detecting vibrations of air. Speaking concretly, vibrations of air causing slight pressure variations on the order of 0.n to 0.0n mmH.sub.2 0 at frequencies of several tens to 0.n Hz in a room are produced, for example, by opening or closing a smoothly working door and vibrations of air causing slighter pressure variations at frequencies on the order similar to those mentioned above are produced by opening or closing a sliding door, but the pressure sensor cannot detect accurately the vibrations of air causing such slight pressure variations at such very low frequencies and has a complicated structure which inevitably requires a high manufacturing cost. In addition, researches are made to improve an acoustic sensor for detecting vibrations of air causing slight pressure variations, it is still incapable of detecting inaudible sound waves at the very low frequencies described above.